VOL. XXVI.] PHILOSOPHICAL TRANSACTIONS. 383 



heath ; for the first report did not come from thence, and the other as an echo 

 from somewhere else, as beyond me, either on the right or left, or from any 

 other part. And I have often observed the same thing, when great guns were 

 fired on the Thames (especially if the air was clear and calm) as the watch-guns 

 morning and evening : after the report had reached my ears, I heard it running 

 along the river a great way, and for several miles echoing back from the banks, 

 mountains, and rocks, that are very numerous on the Kentish coast. Now all 

 this my friend would ascribe to the repercussion of the houses near me. But 

 not to mention the weakness of the sound, after running over a great many 

 miles, and its incapacity, had it come so far, of being reflected by phono- 

 camptic objects near the observer, rather than by those near the sonorous body, 

 I shall give an instance or two, whence it plainly appeors, that an echo caused 

 by reflecting objects near the sonorous body, may be heard for several miles, 

 equally as well as the primary sound, and sometimes be more intense than it. 

 I have often observed that great guns fired in the evening on the Thames, about 

 Deptford and Cuckold's Point, gave generally a double, triple, quadruple, &c. 

 report, and that the latter reports were the loudest. And when I have gone to 

 either hand, a furlong, or even a quarter or half a mile, still the sound conti- 

 nued the same. Several great guns were fired somewhere between Deptford 

 and Cuckold's Point from a ship, which I saw on the Thames from Upminster 

 church ; the report was five or six times repeated in this manner, pi. 10, fig. 18. 

 Between the flash and the report I reckoned 122 half seconds, the wind blow- 

 ing transversely ; therefore at that time the guns were distant from me upwards 

 of 1 3 miles. The first two reports were weaker than the third, but the last 

 was the loudest of all. When I went a quarter of a mile to the right hand, 

 tha sound was repeated in the same manner, as also when I went to the left : 

 and in some of my stations I plainly heard, besides the repetition of the sound, 

 a languid echo reflected from Upminster church, or the adjoining houses ; and 

 this I frequently observed then upon firing the guns. Also, discharging a great 

 gun somewhere on the Thames, either on this ok the other side of Gravesend, 

 the report was repeated eight, nine, or ten times at least, according to this mea- 

 sure of time in fig. IQ. Many persons took this repetition of the report to be 

 the firing of several guns in a sea-fight ; but I supposed it to be no other than 

 a polyphonous echo, proceeding from the explosion of one or two guns, 

 reflected either from several adjoining ships, or from the shore ; and what con- 

 firms this was, that I not only heard it when walking in the garden, but like- 

 wise several others, who were at a distance ; and Mr. Barret heard the same 

 repetition of the report at his house, about 4 miles from Upminster. 

 ^3, Of an Echo, or the Repercussion of Sound in the Air. — When I heard the 



